Sintered iron article



Patented-lune F8, 1946 SIN TERED IRON ARTICLE Alfred L. Boegehold and Robert H. Terry,

Detroit, Mi 1!.

No Drawing. Application September 13, 1943, Serial No. 502.193

, 3 Claims. 1

This invention relates to iron powder mixtures and to the formation of sintered articles i'rom iron powder mixtures. This application is a contlnuation-in-part of our cope'nding application Serial No. 352,522, filed August 14, 1940, now U. 8. Patent 2,382,601.

Iron powder mixtures in accordance with the invention may be used for a variety of purposes and are particularly advantageous in the formation of briquetted and sintered iron articles. Articles formed of briquetted and sintered iron powder may replace articles formed by more conventional methods from cast iron or steel.

In the formation of the sinte'red articles the iron powder may be compressed or briquetted into substantially the exact shape and size desired andin this way expensive machining operations are dispensed with. The pressure used in the briquetting operation is usually within the range of about 30,000 to 100,000 pounds per square inch. Suitable binders, fluxes, die lubricants or pore forming materials may be mixed with the iron powder and compressed therewith to the desired shape. The briquetted article is then heated to a sintering temperature. Temperatures of around 2000 F. to 2100 F. are suitable. Preferably the sintering Operation is carried out under non-oxidizing conditions, as in an atmosphere containing carbon monoxide'or other reducing gas or gases.

An important object of the invention is the when briquetted and sintered has important provision of an improved iron powder mixture particularly adapted for formation of sintered iron articles. Other objects of the invention are: to provide an improved sintered iron article, to provide an improved iron powder mixture, to provide an improved sintered iron article free of objectionai impurities, and to provide an economical iron powder mixture which may or may not have special alloying elements therein and which is particularly adapted for formation of sintered iron articles. Other objects and advantages of the invention will become more apparent as the description proceeds.

The iron powder mixture of the invention is composed of an intimate mixture of solid, irregularly shaped iron powder particles an porous, spongy irregularly shaped iron powder particles. It has been found that such powder mixture has important properties whereby it is particularly adapted for forming sintered articles and particularly so where the briquette is to be impregnoted with molten'metal or other pore filling maF terial. The solid, irregularly shaped iron powder particles may conveniently be formed bypulverizinp or comminutins a suitable iron-carbon alloy and thereafter decarburizing the iron-car bon alloy. Preferably steel, such as high carbon steel, is employed as the iron-carbon alloy. Cast iron, especially white cast iron, may also be employed as the iron-carbon alloy. The porous, spongy irregularly shaped iron powder particles may be conveniently formed by pulverizing a material such as mill scale to a very fine size powder and thereafter reducing the mill scale powder to iron powder without melting. The size of the iron powder in any particular case depends on the use to which the powder is to be put. Or-Hw y in the formation of briquetted and sintered mrts of the iron powder it is preferred that the powder be of such fineness that it will all or substantially all pass through a'200 mesh screen or sieve. Sizes both larger and smaller than this may be used also.

It has been found that a mixture of reduce mill scale powder and decarburized iron powder characteristics not found in sintered briquettes formed of the separate components, making up the mixture, which characteristics may be made use of where it is desirable or use to control the size and shape of pores in the sintered shape. By selecting and choosing mixtures of the powders a wide range in size and shape or the pores in the tered article may be produced. This control of porosity is of especial advantage where it is intended to impregnate the sintered object with a pore filling material such as molten metal, oil or other ingredient. It appears, contrary to what might be expected, that briquetted and sintered articles formed entirely of a spongy. porous iron powder, such as that obtained from reduced ii-1 scale, have thin voids or pores of such shape that it is difiicult, if not impossible, to impregnate the article satisfactorily, especially where-the impregnant is a molten metal such as lead. s dculty appears due to surface ten sion. On the other hand, it appears that sintcred articles produced entirely of an iron powder such as produced by decarburizing the steel or cast iron powder and which consists of solid, irregularly shaped particles, has fat voids of such size and shape as to be relatively easily impregnated. Thus by utilizing suitable mixtures of the two powders the size and shape of the pores in the sintered article may be controlled so as to pro- .of the impregnating component are necessary.

One method f making the mixture of iron powders is as follows: Mill scale is ground or comminuting particles of suitable iron-carbon alloys such as steel or cast iron, especially white cast iron. The mixture of finely powdered mill scale and powdered iron-carbon alloy is then treated under such conditions that the carbon in the iron-carbon alloy powder combines with the oxygen in the mill scale to reduce the mill scale to iron and the iron-carbon alloy is decarburized.

An economical source ofthe iron-carbon alloy powder is the dust collected from shot blasting operation which is largely formed by the breakdown of the white cast iron shot used. Dust or powder produced in this manner requires no further grinding or comminuting.

The iron-carbon alloy powder may be produced, also, by atomizlng iron containing a desired amount of carbon and comminuting the resulting product. The atomizing step may be carried out by flowing a stream of the molten iron-carbon composition into a high velocity jet or jets of water and/or steam and/or air. The brittle and rather small particles produced in this manner may be thereafter ground or comminuted to the size of powder desired.

Where the iron-carbon alloy is to supply all of the carbon required to reduce the mill scale to iron, the amount of carbon in the iron-carbon alloy is determined by analysis and the quantity of the powdered'iron-carbon alloy to be mixed with the powdered mill scale is so chosen that a sufficient excess of carbon is present over that required to combine chemically with the oxygen in the powdered mill scale. In this way the mill scale is reduced to iron when heated in a furnace at a temperature high enough to promote the remill scale be ground very duction of the oxides of iron without melting, and

in a gas atmosphere containing reducing gases in excess of oxidizing gases. Such temperatures may be in the range of about l400-2000 F. The resulting iron powder may contain a small amount of carbon.

The ratio of mill scale to the iron-carbon alloy powder is. of course, rather low if the iron-carbon powder is to provide all the carbon required to reduce the mill scale to iron. However, if desired, some of the carbon required may be supplied !rom the furnace atmosphere and in this way satisfactory iron powder has been produced with astarting mixture composed of equal parts of powdered iron-carbon alloy and powdered mill scale.

The reaction initiated by the intimate contact of the powdered mill scale and of the powdered iron-carbon alloy in the mixture is completed with the aid of the reducing atmosphere surrounding the mixture. The form of carbon available in the iron-carbon alloy powder presumably reacts more readily with the furnace gases than would some form of solid carbon. It is probable that the carbon in the iron-carbon alloy powder first combines with C0: or H2O in the furnace atmosphere to form CO or CO and nascent hydrogen, both of which react with oxygen in the iron oxide to reduce the latter to iron. The resultant powder is composed of a. mixture of spongy, porous irregular particles from the reduction of the mill scale and solid irregular v4 shaped particles resulting from the decarburization of the iron-carbon alloy particles.

By mixing the constituents required for the reducing reaction, reduction takes place uniformly throughout a thick layer of the powder layer. Consequently the output rate of the furnace used is high. The formation of the nascent reducing gas in close proximityto the place where it is needed is considered an important advantage derived from treating a mixture of powdered mill scale and powdered iron-carbon alloy. The best results are obtained when the mill scale is ground to 250 mesh or iner.

In certain instances iron-carbon alloy particles to be mixed with the powdered mill scale may contain a desired and regulated amount of special alloying constituents.

For example, manganese, molybdenum, vanadium, nickel. titanium, copper, etc. in small amounts may be useful in special applications. In forming articles of sintered iron powder which are to contain special alloying constituents it is desirable that the alloying elements be already alloyed with the iron. This may be done conveniently in accordance with this invention by forming a molten iron-carbon alloy containing the desired amount of the alloying element or elements and atomizing and comminuting the resulting particles. Certain alloying elements may be added, also, to the iron from which are formed the shot for use in shot blasting operations. Theworn out shot will thus contain desired alloying elements.

The mill scale is quite brittle and friable and is easily comminuted or ground to a very fine powder. For this purpose any suitable pulverizer, crusher or ball mill may be used.

Due to the fact that it is important that the fine, it may not be necessary to grind or comminute the iron-carbon alloy to produce powder of the same degree of fineness, as the finely powdered mill scale would make up the percentage of very fine powder needed in the making of sintered iron articles. The spongy particles resulting from the reduction of the mill scale assist in causing coherence of the particles in the briquetting operation.

The following are specific examples for illustrativepurposes. of iron-carbon alloy powder compositions that may be used.

Example No, 1

Percent 3.14 Manganese .41 Silicon 1.53 Balance iron plus minor amounts of ordinary impurities such as phosphorus, sulphur, etc.

The analysis of Example No. 1 is that of white iron shot used in shot blasting operations.

Carbon The composition of Example No. 2 is one that is especially adapted for making the iron-carbon alloy powder by the atomizing and grinding method described herein.

As illustrative of Carbon a reducing gas atmosphere it is contemplated that the Percent Co -11 Hz 13-16 CO2 4 5 Na the'balance The two kinds of powders making up the mixture of iron-powders may, of course, also be produced separately and thereafter mixed in the desired proportions.

Various changes and modifications in the embodiments of our invention described herein may be made by those skilled in the art without departing from the principles of our invention.

We claim:

1. A porous ferrousmetal product comprising a compacted and sintered mass composed essentially of an intimate mixture of spongy, porous particles of substantlally pure iron resulting from reduction without melting of pulverized mill scale of a size such that it will at least substantially all pass through a screen of about 250 mesh, and solid, irregularly shaped iron powder particles of small size resulting from the substantially comaeoaiao plete decarburization of a powder of the class of ferrous metal powders consisting of steel and cast iron powders.

2. A porous ferrous metal product comprising a compacted and slntered mass comprising an intimate mixture of spongy, porous particles of substantially pure iron resulting from reduction without melting of pulverized mill scale of a size such that it will at least substantially all pass through a screen of about 250 mesh, and solid, irregularly shaped iron powder particles of small size resulting from substantial decarburization of .steel powder particles.

burization of a powderof the class consisting of steel and cast iron powders.

ALFRED L. BOEGEHOLD. ROBERT H. TERRY. 

